Starting two rigid body animations at different points in time
Select the pen then go to the frame in which you don't want the pen to fall (presumably frame 1) and uncheck the Dynamic tick in the rigid body menu. Then mouse over the tick and press I.



This adds a key-frame. Then go to the frame where you want it to fall, and check the Dynamic box (so it's on) and press I to add another key-frame. Then the object will have dynamic rigid collision when the key-frame is hit.